Thursday, May 5, 2016

Darwin the Newtonian. Part I. The Darwinian worldview

History shows that, even in science, things that everyone has long taken for granted may not be true.  Thinking in ways more carefully constructed to be restrained by what we actually know is often difficult, and the temptation is to believe what we want to believe. There are many normal, human, not to mention professional reasons for this.  But it's not good for science.  What may appear to be clear-cut 'objective' concepts about the material world can verge on the abstract or even philosophical, based on subjective opinion more than fact.  As we've discussed before, in a sense this is due to our need in evolutionary biology to rely on statistical tests based on internal comparisons, rather than to use statistical methods to test hypothesized, externally derived laws of nature (see this series and earlier--search on 'statistics' or 'p-value').

In 1859, Charles Darwin's Origin of Species culminated what considerable contemporary rumination had been suggesting, with his assertion that life today is the result of a material, historical process, by which current organisms have arisen by divergence from a common ancestry.  His synthesizing insight transformed biology in many ways.  Before that biology had largely been a descriptive science.  Before Darwin, with a few very speculative exceptions, the best causal explanations for the diversity and adaptations of organisms had been that God created them on an ad hoc basis.  Darwin saw otherwise, but his thinking was embedded in his era's general views about science.

Thanks to developments in the European Enlightenment period, by Darwin's time causation in nature was being viewed, by scientific thinkers at least, as based upon natural laws.  The Newtonian view of the cosmos was the prevalent one and, in keeping with this, Darwin adopted an implicitly quantitative, law-like view of biology.  As far as I know, Darwin was not a diligent student except in relation to areas like geology and botany, and he certainly was not mathematical (he himself said so).  However, he must have known at least something about Isaac Newton (a rather famous Cambridge predecessor).

Isaac Newton; 1689 by Godfrey KnellerWikipedia

Still, whatever he formally knew of Newton's laws of motion Darwin essentially accepted some of Newton's basic laws of motion, which we can state as follows:
1.  An object at rest remains at rest (law of inertia)
2.  Objects move or change motion only when force-like acceleration is applied, (and the greater the mass of the object the greater the force needed to change its motion)
3. Every action involves an equal and opposite reaction (when pushed, an object pushes back)

There are, I think, important analogs in Darwin's thinking, and there is still today widespread uncritical application of Newtonian-like thinking to Darwin's ideas.  The other day, I heard a deservedly famous and prominent geologist say that Darwin's 'Null hypothesis of evolution' was that unless the environment changes, no evolution will occur. This is analogous to the law of inertia, and I think it's actually fundamentally quite wrong, but we will see why it seems tempting and plausible.

The classical idea, still asserted without much if any questioning, is that organisms are fitted to their environment.  Analogous to the Newtonian law of inertia, if the environment doesn't change, neither will the organisms.  Darwin was, to my knowledge, not wholly explicit about this, but it was at the very least implicit in his view as expressed in The Origin of Species.  At least, by the 6th edition he recognized that there can be long time periods when organisms seemed not to change.

However, and this is analogous to Newton's second law, if the environment changes, then in a force-like way it screens the varying genomes of organisms, favoring those that are suited to the new conditions.  The force Darwin called natural selection.  I'm mixing bits of new and Darwin-time terminology here, but the gist of Darwin's view is that natural selection is a deterministic force, which he likened to the force of gravity in his law-like, deterministic worldview in regarding to 'motion' (change) in organisms.  Indeed, he many times asserted that the smallest difference among organisms would be detected and screened by selection.

After this has gone on for a while, the selective 'acceleration' ceases because the organisms are now adapted to their surroundings.  At that stage, the law of inertia takes over. His theory of inheritance was fundamentally wrong, but the Darwinian idea expressed in modern genetic terms is that the organisms in a population at any time and place vary genetically, and when the environment changes, those whose genotypes are best suited to the new environment will reproduce more prolifically, and will increase in frequency, driving inferior genotypes out of the population.

The Darwinian analogue to Newton's third law of motion is that changes in the nature of one organism in a local area improves its use of, and thereby alters, its local ecology.  The faster foxes catch the rabbits and proliferate. But this in turn makes the rabbits hoppier.  This then sets up a new force--in the local organisms--that Darwin referred to as the relentless 'struggle for existence.'

There are some issues in this view that are not well enough appreciated.  Darwin's endless struggle for existence suggests a continuing maelstrom of change, and yet it has been noticed that some species, based, for example, on ancient fossils.  Likewise, widely dispersed species that seemed similar across their areas of habitation implied that they had long had been static--because it takes a long time to spread over vast geographic areas.   In the case of some dinosaurs, a hundred or more million years, and based on some bacterial fossils, several billion.

Stromatolite (bacterial fossil); Western Australia, By Didier Descouens 

The idea this suggests is one of evolutionary stasis. This was recognized by Darwin, at least by the 6th edition of the Origin, and he mused over how periods of stasis would lead eventually to evolutionary change.   This idea, often now called 'punctuated equilibrium,' was claimed by Gould
in his final tome to be his own life's main discovery and contribution.  Perhaps he had not read Darwin closely enough?

An important point here is to recognize what Darwin was trying to account for.  Either selection is a relentless force-like aspect of nature, or there can be a static period when no force is being applied. How can both be true?

One answer is that there is no way for genotypes to be static, because mutations always arise.  Even if some are purged by selection's force, many will be selectively neutral and genomic evolution will always be occurring.  However, what we can see in fossils is only some aspects of morphology.  This means that while genomes are evolving, at least neutral parts, some aspects of traits persist, for adaptive or whatever other reason.

The idea of an evolutionary 'Null hypothesis' is hence elusive.  In one sense, some trait may not change unless the selective environment changes.  In another sense, selection can maintain functionally adaptive traits, while other traits and neutral DNA sequences change.  The traits may not 'evolve', but the sequence does.

Such ideas go against even Darwin's idea of life as an endless universal struggle, and perhaps why he had to do some rationalizing to account for apparent stasis.

Even this account for stasis of a single species would seem incompatible with the view of a relentless struggle among species that drives all of them in the endless rat-race of adaptation.  In that reality every part of an ecosystem affects every other part, so how can there be stasis?

We will think about some of these issues in the next three posts.  First, we'll ask whether life really can be viewed as 'Newtonian,' as Darwin did.  Then, we'll ask whether natural selection and genetic drift actually exist as they are universally characterized to be.  We'll see that our theories and our methods of inference, leave major issues open even about these fundamental aspects of the theory of life.

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